This invention relates to a method and device for correcting an error in high efficiency coded digital data such as data used in a Digital Compact Cassette (DCC) system and a Mini Disc (MD) system and, more particularly to a method for correcting an error in digital data in a case where data of one frame is totally irremediable when an error has occurred.
In the DCC and MD systems, no data of digital audio signals, which are sampled at a predetermined frequency and are quantized at a predetermined bit number, are recorded directly in recording media as in a Compact Disc (CD) or in a Digital Audio Tape (DAT) but a signal component only is recorded after being subjected to a highly efficient coding processing.
Details of the DCC system etc. are described in "NIKKEI ELECTRONICS" No. 535 (Sep. 2, 1991), P.127-P141, "NIKKEI ELECTRONICS" No. 542 (Dec. 9, 1991), P.125-P. 168, and "RADIO GIJUTSU", December 1991, P.156-P. 161.
FIG. 4 is a block diagram schematically showing a DCC recording device for example.
An analog audio signal is converted to digital data Of, e.g., 16 bits at 48 KHz, by an analog-to-digital converter 41 and then divided into signal components of 32 frequency bands by a subband filter 42. Thus, data of 32 frequency bands.times.12 samples from the subband filter 42 constitute a signal processing unit to be processed at a time. If the sampling frequency is 48 KHz, unit time for processing one signal (one frame) is 32.times.12/48K=8 msec.
The data from the subband filter 42 is supplied to a bit assignment data generation section 43, a mantissa data generation section 44 and an exponential data generation section 45. The bit assignment data generation section 43 performs assignment of bits ranging from 2 bits to 15 bits, for example, adapted to human hearing characteristics to plural data on the basis of signal patterns of 32 frequency bands. The exponential data generation section 45 quantizes a scale factor portion, which is the largest data in the 12 sampled data, of the output of the subband filter 42 at a predetermined number of bits, e.g., 6 bits, whereas the mantissa data generation section 44 quantizes a mantissa portion of the output of the subband filter 42 at a number of bits which has been assigned by the bit assignment data generation section 43.
These data are converted to data of the PASC (Precision Adaptive Subband Coding) format by a format generation section 46. The data of this format is shown in FIG. 5. Data of one frame (8 msec) consists of a synchronizing signal, bit assignment data, exponential data and mantissa data. Total data amount of one frame is constant but data of subbands existing in the audio range only are bit-assigned and the bit-assigned data only are stored in correspondence to bit numbers designated by the bit assignment data and, therefore, data amount of exponential data and data amount of mantissa data in each subband differ depending upon the subband and frame.
Audio data converted to the PASC format is imparted with a code for CIRC (cross interleave Read-Solomon code) correction by an error correction and encoding section 47 and is recorded on a recording medium after being subjected to 8 to 10 modulation.
Although total data amount of one frame is constant in this system, no data is recorded in either the exponential section or the mantissa section in a subband to which a number of bits has not been assigned. Even in a subband to which a number of bits has been assigned, the data amount of the mantissa section differs depending upon the subband and frame.
In this system, therefore, when an error has occurred in frame data during reproduction of data and this error cannot be corrected by the error correction means, correction cannot be made simply by employing a linear correction method as currently employed in the CD or DAT system. When, for example, an error has occurred in bit assignment data, all data of one frame becomes totally unidentifiable. This is because data position in one frame undergoes change in accordance with the bit assignment data. Therefore, when an error has occurred in the bit assignment data, all data of one frame must be copied from a preceding frame with resulting serious deterioration in a reproduced tone.
It is, therefore, an object of the invention to provide a method for correcting an error in digital data which is capable of preventing deterioration in reproduced tone even when an irremediable error has occurred in the digital data.